The present invention is directed to bicycle crank arms and, more particularly, to a hollow bicycle crank arm that uses a removable sand mold during casting.
It is desirable for a bicycle to be as lightweight as possible, so the bicycle parts should be reduced in weight as much as possible. This is true of bicycle cranks as well. A bicycle crank that is made lightweight by being manufactured in the form of a hollow tube is known from Japanese Patent Publication 2-18652, for example. Furthermore, a method for forming an internal cavity in a solid material by extrusion forging has also been proposed in Japanese Laid-Open Patent Application 5-116670. This hollow crank is obtained by the welding or plastic deformation of a pipe or crank billet, but this method affords little freedom in the design of the crank shape. The shape is further restricted because molding is impossible without certain portions that are otherwise unnecessary in terms of material dynamics, among other reasons. Another drawback is that a high quality appearance is difficult to achieve because of limitations in the machining process, despite demand for certain types of designs.
Methods for manufacturing a bicycle crank from a light alloy by casting are also known from Japanese Laid-Open Patent Application 58-93554. The shape restrictions noted above are eliminated with these casting methods, but forming a cavity on the inside is difficult with a crank because of the small size of the part, and the hollow interior can degrade the mechanical strength of the product Accordingly, it has been proposed that a pipe or the like be integrally cast in the interior as shown in Japanese Laid-Open Utility Model Applications 48-7948 and 61-131391. Integrally embedding a pipe or other such member with high strength in the crank does indeed preserve the strength of the crank, but a problem remains in terms of making the product lightweight.